1. Field of the Invention
This invention relates to selected novolaks prepared via a two-stage process which contain high levels of ortho--ortho methylene bridge bonding. In the first stage, a low molecular weight oligomeric prepolymer with high ortho--ortho bonding is formed via a noncatalyzed reaction of phenolic monomers with an aldehyde source under elevated temperatures and pressures. In the second stage, the oligomeric prepolymer is extended to higher molecular weights to obtain a novolak resin with desirable alkaline solubility, molecular weight and ortho--ortho bonding properties. These novolaks can also be fractionated to further improve their lithographic properties.
Furthermore, the present invention relates to radiation-sensitive compositions useful as positive-working photoresist compositions, particularly those containing these phenolic resins and o-quinonediazide photosensitizers. Still further, the present invention also relates to substrates coated with these radiation-sensitive compositions as well as the processes of coating, imaging, and developing these radiation-sensitive mixtures on these substrates.
2. Description of Related Art Including Information Disclosed under 37 CFR .sctn..sctn.1.97-1.99
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate. In some instances, it may be desirable to bake the imaged coated substrate after the imaging step and before the developing step. This bake step is commonly called a post-exposure bake and is used to increase resolution.
There are two types of photoresist compositions--negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g., a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the nonexposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g., a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of one-half micron or less is necessary.
In addition, it is generally desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.
Increased resolution has been noted in positive photoresist systems whose novolaks possess a high degree of ortho--ortho bonding. The term ortho--ortho bonding is used to refer to the location and positions of attachment of the methylene bridge between phenolic nuclei. Thus, the bridge which connects two phenolic nuclei which is ortho to both phenolic hydroxyl groups is regarded as ortho--ortho.
It is thought that ortho--ortho bonding increases the interactions between the novolak and the photoactive compound in positive photoresists compared to positive photoresists containing novolaks which lack a high degree of ortho--ortho bonding in their microstructure. Although the exact character of these interactions is speculative, e.g., hydrogen-bonding, van der Waals forces, and the like, there is a correlation between increased resolution and contrast observed in these positive resists whose novolaks contain a high degree of ortho--ortho bonding compared to positive resists whose novolaks lack this high degree of ortho--ortho bonding.
In a conventional novolak system such as m-/p-cresol novolak, as the p-cresol content is increased, the ortho--ortho bonding increases. This leads to increased inhibition and discrimination and ultimately results in higher resolution. However, beyond a certain level, the extra p-cresol typically results in scum formation between resist images and a slowing of the photospeed. This renders the photoresist unusable.
European Patent Application 0118291, published on Dec. 9, 1984, and Hanabata et al., SPIE Vol. 631, pages 76-82 (1986) describe the synthesis of preparing high ortho--ortho novolaks by polymerizing m-cresol and formaldehyde with a divalent metal catalyst. This method circumvents the above-noted tradeoffs which exist with novolaks made from high content p-cresol phenolic mixtures.
Separately, Casiraghi et al. (makromol. Chem. 182 (11), 2973, 1981 describes a procedure to prepare high ortho--ortho novolaks by a noncatalyzed process. However, this paper did not report using this procedure with m-cresol or a mixture of m-/p-cresol monomer feeds.
The present invention differs from these teachings and prepares unique high ortho--ortho novolaks which are mainly made from phenolic monomer feeds which are mainly trifunctional (e.g., m-cresol or 3,5-xylenol). These high ortho--ortho novolaks which when formulated into resists, impart high resolution and sensitivity.
The prior art also replete with numerous other references which teach the making of specific novolak resins useful in radiation-sensitive mixtures. Such references include U.S. Pat. Nos. 4,377,631 (Toukhy); 4,529,682 (Toukhy); 4,587,196 (Toukhy); 4,837,121 (Blakeney et al.); 4,959,293 (Blakeney et al.); 4,970,287 (Blakeney et al.); 5,053,479 (Blakeney et al.); 5,024,921 (Blakeney et al.); and 5,196,289 (Jeffries et al) as well as published PCT Patent Applications Nos. PCT/US90/04307 (Jeffries et al.); and PCT/US90/03601 (Ebersole).